In the recent years, accompanied with the fast-developing technology from computerization, networking to the current digitization, operation modes of various industries are inevitably changed as well. Among the changes, digitalization of the media industry is considered as one of the most important and revolutionary tasks that entrepreneurs look into. The associated digital television (DTV), being a center of attention in terms of consumer electronics, is not only a focus in the development of information appliances but also a critical gateway for the Internet to enter the living room of households.
A common DTV adopts Digital Video Broadcasting (DVB) signals of the DVB standards for transmissions of various television signals. The core structure of DVB is an orthogonal frequency division multiplexing (OFDM) modulation system that performs wireless signal transmissions.
In an OFDM system, data is modulated into signals at a transmitting end, and is then transmitted over channels by such as quadrature phase-shift keying (QPSK), quadrature amplitude modulation (QAM) and phase-shift keying (PSK). The OFDM system is capable of supporting large-amount data transmissions, and has greater usage flexibility in bandwidth. However, the OFDM system is not entirely flawless; for example, when applied in wireless transmissions, the OFDM system is resulted with timing offset, phase offset and frequency offset. These issues have great impacts on received signals at the receiving end, and the undesirable effects substantially reduce performance of the OFDM system. In a real environment, channels of the OFDM system are changed along with the environment and time. When transmitted to the receiving end via wireless channel transmission, received signals at the receiving end may be different from the ones transmitted from the transmitting end for that the signals are prone to distortion due to changes or interferences of the channels. At the receiving end, in order to recover the received signals from distortion, the effects of the channels need be first estimated to accurately; that is, channel estimation need first be performed accurately recover the signals transmitted from the transmitting end. Common channel estimation is implemented using pilot signals. More specifically, a plurality of pilot signals are placed in sub-channels of specific frequencies at the transmitting end, and the known pilot signals are then used at the receiving end to calculate channel responses of the sub-channels of the specific frequencies.
FIG. 1A shows a block diagram of a common receiving end comprising a synchronizer 11, a channel estimator 13, an equalizer 15 and a decoder 17. The synchronizer 11 receives a wireless signal 10, and generates a synchronization signal 12. The synchronization signal 12 is compliant to, for example, Digital Video Broadcasting-Terrestrial (DVB-T), Digital Video Broadcasting-Satellite (DVB-S), Digital Video Broadcasting-Cable (DVB-C) standards specified by the DVB standards, digital television standards specified by the Advanced Television System Committee (ATSC), or associated digital television standards specified by other standard organizations.
The channel estimator 13 receives the synchronization signal 12, and performs calculations and estimations for pilot channel responses in the synchronization signals 12 according to known pilot signals 14. The equalizer 15 receives channel responses 16 estimated by the channel estimator 13, and processes the synchronization signals 12 according to the channel responses 16 to generate equalization signals 18, which is regarded signals transmitted by the transmitting end. The decoder 17 then receives and processes the equalization signals 18 to generate digital television signals to be played.
It is apparent from the above description that, the synchronization signals 12 transmitted by the transmitting end may be accurately recovered, provided that the channel responses needed during the recovery are obtained according to the channel responses of the pilot signals.
FIG. 1B shows a relationship diagram between frequency and time of DVB signals having pilot signals; note that only partial sub-channels and time points are shown. In FIG. 1B, the horizontal axis f represents sub-channels of different frequencies, and the vertical axis t represents different time points. The channel response in a frequency f sub-channel at a time point t is represented as H (frequency f, t time), and the receiving time is from the negative to the positive. For example, a signal received at a time point t=−1 is before a signal received at a time point t=1, and a signal received at a time point t=4. Pilot signals are transmitted over specific frequency sub-channels with DVB signals transmission, and are categorized into at least continuous pilot (CP) signals and scatter pilot (SP) signals. The continuous pilot signals are placed at all time points over a specific frequency sub-channel (or referred to as a continuous pilot sub-channel). For example, pilot signals are placed at all time points at coordinate axes f=−3 and f=27. The scatter pilot signals are placed intermediately over a specific frequency sub-channel (or referred to as a scatter pilot sub-channel). For example, at the coordinate axis f=0, pilot signals are placed at time points t=−4, 0, 4, and so on. That is, pilot signals are placed at coordinates (0, −4), (0, 0), (0, 4), and so on. For another example, at f=3, pilot signals are placed at time points t=−5, −1, 3, and so on; that is, the coordinates (3, −5), (3,−1), (3, 3), and so on. The transmitting end transmits and the receiving end receives the continuous pilot signals and the scatter pilot signals in compliance with DVB standards. Therefore, the receiving end determines distortion of the DVB signals transmitted through the channels according to the continuous pilot signals and the scatter pilot signals, so as to increase accuracy in recovering the signals. Further, channel frequency responses of the DVB signals at time points without pilot signals over the scatter pilot sub-channels are calculated and estimated. Up to this point, all sub-channel responses of the OFDM communication system are obtained. However, most of the channel responses are obtained on basis of estimation that inevitably contains certain degrees of errors. Particularly, under circumstances that the receiving end is moving at a high speed, and more severe inaccuracies in channel estimations are incurred and result in erroneous signal recovery.
Therefore, the invention provides a new channel estimator and a channel estimation method to estimate more accurate channel responses for overcoming the foregoing issues.